In general, MySQL is one of the faster solutions with a smaller storage
footprint, and is well suited for both small and large-scale implementations.

The hash driver (inspired by Bill Yerazunis' CRM Sparse Spectra algorithm)
is the fastest solution by far and requires no dependencies, supports
an auto-extend feature to grow the file size as needed, and is very
fast and compact. It does, however, lack some features (such as merged
groups support) and uses a lot of memory to mmap() users.